The present invention relates to a wastewater treatment method and apparatus for treating organic wastewater having oil and fatty oil contents.
Generally, organic wastewater of high concentration is discharged from many industrial facilities, research laboratories, and the like. When treating such high concentration organic wastewater with a typical conventional activated sludge, it has been usual practice to dilute the high concentration organic wastewater to facilitate treatment thereof in case where inflow high concentration organic wastewater is of too high concentration for treatment with the activated sludge. However, such practice of diluting high concentration organic wastewater in treating the wastewater involves a disadvantage that a wastewater treatment apparatus of greater size is required, which means increased capital expenditure. Therefore, a need exists for a compact wastewater treatment apparatus which can treat high concentration organic wastewater without dilution being required.
In such a situation, a more recent practice in the art of wastewater treatment is to utilize submerged membranes. The term "submerged membrane" herein means an ultra-filtration film or membrane or a precision filtration film or membrane as disposed in tank water for use in wastewater treatment.
By using such a submerged membrane it is possible to carry out wastewater treatment in such a condition that the concentration of microorganisms within the aeration tank is increased up to two times to six times the microbial concentration level usual in the case of a conventional activated sludge process. This not only makes it possible to provide a compact arrangement for wastewater treatment, but also enables high concentration organic wastewater to be effectively treated without being diluted.
Generally, in biological wastewater treatment under the activated sludge process which utilizes microorganisms, inflow water is pretreated for water quality adjustment in respect of quality factors, such as pH, BOD (biochemical oxygen demand), and COD (chemical oxygen demand), thereby to facilitate biological treatment.
In biological treatment, it is important to pretreat the water to be treated in such a way as to facilitate treatment by microorganisms. If pretreatment is not carried out, adequate microbial treatment capability cannot be obtained, and there may often occur an abnormal phenomenon such as bulking (abnormal growth of stringy microorganisms on the activated sludge which hinders sludge settlement) and so on, which is peculiar to microbial treatment operations.
As earlier stated, prior art pretreatment includes chemical neutralization for pH adjustment, and dilution for adjustment of BOD and COD.
A wastewater treatment apparatus of the type shown in FIG. 5 is generally known for treating with microorganisms high concentration organic wastewater in which suspended solids are present but in an extremely small amount. This apparatus includes an anaerobic tank 101 and an aerobic tank 102. Water to be treated is first anaerobically treated in the anaerobic tank 101. Shown by 103 is a submerged agitator. The water which has been anaerobically treated in the anaerobic tank 101 is then caused to flow into the aerobic tank 102. In the aerobic tank 102, the water is aerobically treated and is allowed to pass through a submerged membrane 6 before it reaches a submerged membrane draw pump 108. Then, the water passes sequentially through a secondary treatment unit 110 and a tertiary treatment tank 111 before it is discharged as treated water. A nutrient is added into the aerobic tank 102 from a nutrient tank 112 through a pump 113. The nutrient is a feed for aerobic microorganisms, Also, a liquid for generating suspended solids (SS) is added into the aerobic tank 102 from a suspension tank 115 via a pump 116. Disposed in the aerobic tank 102 is a submerged pump 117 which has a function to return sludge settled on the bottom of the aerobic tank 102 to the anaerobic tank 101 and a sludge treatment unit 118.
In a wastewater treatment apparatus of the type shown in FIG. 5 wherein water is treated by means of a combination of aerobic tank and anaerobic tank, the anaerobic tank has an important position. In particular, it has been found that in a wastewater treatment apparatus such that submerged membrane 106 is disposed in aerobic tank 102, as shown in FIG. 5, conditions for treatment in the anaerobic tank 101 is a factor having great bearing upon the permeation efficiency of the submerged membrane 106. In other words, oily substances contained in the water to be treated must be thoroughly anaerobically treated in the anaerobic tank 101, otherwise the permeation efficiency of the submerged membrane 106 in the aerobic tank 102 is seriously lowered. Such possible decrease in the permeation efficiency of the submerged membrane 106 remains as a problem yet to be solved.
As a more recent development, various kinds of wastewater treatment apparatus and sludge treatment apparatus have been proposed including those disclosed in Japanese Patent Application Laid-Open Nos. 3-232597 and 4-313400.
The sludge treatment apparatus disclosed in JP Laid-Open No. 3-232597 represents a proposal for solving the problem of possible decrease in permeation efficiency due to adhesion to an ultrafiltration membrane of oily matter contained in sewage sludge such that a coagulant is added into sewage tank for removing such oily matter. However, the use of a coagulant involves increase operating cost and, in addition, sludge generation.
Another problem is that in the FIG. 5 apparatus and JP 3-232597 apparatus, water to be treated is introduced into the anaerobic tank in such a condition that dissolved oxygen content of the water remains untreated, with the result that the anaerobic tank is not in a condition favorable to anaerobic microorganisms. In other words, when water to be treated which contains dissolved oxygen is introduced into the anaerobic tank 101, the anaerobic atmosphere is rendered incomplete in a portion of the anaerobic tank 101, with the result that anaerobic microbial activity cannot be actively effected.
More specifically, by way of example, problems with microbial treatment of developer liquor containing wastewater, as high density organic wastewater, discharged from a semiconductor plant or liquid crystal plant, will be discussed in detail. Such developer liquor containing wastewater contains Tetramethyl Ammonium Hydroxide (hereinafter referred to as TMAH) which exhibits biotoxicity on the order of 2000 to 10000 ppm. The developer liquor containing wastewater contains, in addition to TMAH, surfactant and resist (photoresist) or colored oily and fatty matter, which are persistent substances to various kinds of microorganisms. Unfortunately, such developer liquor containing wastewater does not contain phosphorus, a substance necessary for microbial propagation, or SS (suspended solid), a material for microbial deposition, at all. Generally, for purposes of microbial treatment, an ideal quality of inflow water (water to be treated) is such that the ratio of BOD (biochemical oxygen demand) to N (nitrogen) to P (phosphorus) is 100:5:1 and that the water contains adequate SS. Therefore, if the water does not contain any phosphorus or SS at all, it is necessary to add phosphorus, as a micro ingredient, and adequate SS to the water to be treated.
In particular, any developer liquor containing wastewater discharged from a semiconductor plant or a similar plant contains BOD and nitrogen, but does not contain phosphorus or adequate SS at all. Further, such developer liquor containing wastewater contains surfactant that is persistent to microorganisms and has foaming characteristics. Examples of such persistent surfactant include alkyl ammonium-family surfactant and polyoxyethylene-family surfactant. It has been experimentally proved that such persistent and foamable surfactant can be treated by exceedingly increasing microbial concentration through utilization of a submerged membrane.
Therefore, when treating developer liquor containing wastewater from a semiconductor plant in the anaerobic tank 101 and aerobic tank 102 with the submerged membrane 106 as shown in FIG. 5, it is necessary that phosphorus, as a nutrient necessary for propagation of microorganisms, be added in the form of phosphoric acid from the nutrient tank 112 into the aerobic tank 112. It is also necessary that a liquid for generating SS, i. e., suspended solid (for example, waste milk powder liquid or the like) be supplied from the suspended solid tank 115 into the aerobic tank 102 in large quantities. Addition of phosphoric acid, as well as addition of a liquid for generating SS, involves increased operating cost.
However, unless such addition of phosphorus acid and SS-generating liquid is carried out, microbial concentration is rendered insufficient, with the result that foaming due to surfactant contained in developer liquor containing wastewater will occur. Further, activated sludge adheres to the resulting bubbles and flows outward of the aerobic tank and, as a result, aerobic treatment is rendered impossible.